In a normal human adult male, the gastrointestinal (GI) tract is approximately 7.5 meters long (25 feet) and it extends from the mouth to the anus. It consists of the upper GI tract and the lower GI tract. The upper GI tract includes the mouth (oral cavity, which includes the salivary glands, mucosa, teeth, and tongue), the pharynx, the esophagus and cardia, and the stomach (antrum and pylorus and pyloric sphincter). The lower GI tract includes the intestine and anus. The intestine is broken down into the small intestine and the large intestine. The small intestine has three parts: the duodenum, the jejunum, and the ileum. The large intestine has three parts: the cecum, the colon (ascending colon, transverse colon, descending colon, and sigmoid flexure), and the rectum. The function of the GI tract is to transfer nutrients and water from the external environment to the body.
Each part of the gastrointestinal (GI) tract performs a specialized function in the digestion of food. Digestion is regulated both hormonally and by the autonomic nervous system. The major hormones that regulate the digestive system (e.g. secretin, gastrin and cholecystokinin) are produced and released by cells in the mucosa of the stomach and small intestine. Both arms of the autonomic nervous system, the parasympathetic nerves and the sympathetic nerves, influence the digestive process. The parasympathetic nerves stimulate secretions and peristalsis, while the sympathetic influence is more inhibitory.
Digestion begins in the mouth with chewing. As the teeth break down the food, saliva moistens it to facilitate swallowing. Saliva also contains a digestive enzyme called amylase, which starts to break down some of the carbohydrates (starches and sugars) in the food. After the food is swallowed, it moves into the pharynx and then travels down the esophagus.
The esophagus is a muscular tube that extends from the pharynx through the diaphragm to connect with the stomach in the abdominal cavity. The body of the esophagus is approximately 18-25 cm long extending from the upper esophageal sphincter to the lower esophageal sphincter. The esophagus is divided into four regions: the cervical esophagus, the upper thoracic esophagus, the mid-thoracic esophagus, and the distal thoracic esophagus (FIG. 3). The upper third of the esophagus consists of striated muscle, and the lower two-thirds consists of smooth muscle. A network of intrinsic neurons, found in the muscle layers lining the lumen of the esophagus, communicates to the central nervous system via the vagus nerve, the adrenergic ganglia, and the celiac ganglia. (FIG. 3). Vagal activity increases esophageal activity. Swallowing is triggered by a signal transmitted by the vagus nerve, and the timing of the swallowing sequence (primary peristalsis) is dependent on nerves intrinsic to the esophagus. Sympathetic innervation is sensory in nature.
Before passing into the stomach, food must pass through the lower esophageal sphincter (LES), a ring of increased thickness in the smooth-muscle layer of the esophagus. The LES relaxes before the esophagus contracts, and allows food to pass through to the stomach. After food passes into the stomach, the LES constricts to prevent the contents from reentering the esophagus from the stomach. Relaxation of the LES is maintained by both muscular and nerve mechanisms. The release of acetylcholine by nerves maintains or increases LES tone. Reflex mechanisms, physiologic alterations, and ingested substances can also affect LES tone. The release of nitric oxide by nerves relaxes the LES in response to swallowing, although transient LES relaxations may also occur independent of swallowing. This relaxation is often associated with transient gastroesophageal reflux in otherwise normal people.
Once food reaches the stomach, the stomach muscles churn and mix it with acids and enzymes, breaking it into smaller pieces. Some substances, such as water, salt, sugars, and alcohol can be absorbed directly through the stomach wall, while most other substances need further digestion and must travel into the intestine before being absorbed. Parietal cells within gastric glands secrete hydrochloric acid (HCl), which makes gastric juice acidic, with a pH less than 2. During a meal, the rate of HCl production increases. Seeing, smelling, tasting, and chewing food sends information through the vagus nerves to the parietal cells, causing them to increase acid production. Stomach distention, hydrogen ion concentration, and peptides send messages through long and short neural reflexes to increase gastrin release, which increases HCl production. Before the food leaves the stomach, it is in the form of a thick liquid called chyme.
The chyme passes from the stomach into the duodenum, the first 20 to 30 cm of the small intestine, and through the small intestine for further digestion and absorption. The inner wall of the small intestine is covered with millions of microscopic, finger-like projections called villi, which are the vehicles through which nutrients can be absorbed into the body. Distention of the intestine sends both hormonal and neural reflex messages to decrease gastrin release, which decreases HCl production in the stomach.
From the small intestine, food that has not been digested and some water travels to the large intestine. By the time food reaches the large intestine, nutrient absorption is nearly finished. The large intestine's main function is to remove water from the undigested matter and form solid waste.
There are a variety of digestive problems that can be encountered along various portions of the gastrointestinal tract. In general, the motility of the gastrointestinal tract is based on intricate neurohormonal interactions resulting in the coordinated movement of the gut. Aberration of such motor function can result in common gastrointestinal motility disorders, such as gastroesophageal reflux disease (GERD) and gastroparesis.
Gastroesophageal reflux is a condition in which the weakness of the esophageal sphincter allows the acidic contents of the stomach to move backward up into the esophagus (FIG. 4). In some cases, gastroesophageal reflux is considered physiologic (reflux in normal individuals). This physiologic reflux occurs several times a day in otherwise healthy individuals without associated symptoms or damage. Gastroesophageal reflux is considered a disease (GERD) when the reflux produces frequent or severe symptoms that can cause damage to the esophagus, pharynx or respiratory tract. Complications of GERD include esophageal erosion, esophageal ulcer, esophageal stricture, and Barrett's esophagus. GERD can be attributed to such factors as transient lower esophageal sphincter (LES) relaxations, decreased lower esophageal sphincter (LES) resting tone, delayed stomach emptying, ineffective esophageal clearance, diminished salivation, potency of refluxed material, and the inability of the esophageal tissue to resist injury and repair itself. A weakened LES and transient spontaneous LES relaxation (relaxation not induced by swallowing) is a common cause of reflux (FIG. 5). Gastroesophageal reflux disease commonly results when the resting LES pressure is too low to resist the pressure within the stomach or when the normal angulation of the esophagogastric junction is lost (e.g. hiatus hernia) (FIGS. 6A-6B′). GERD is generally treated by lifestyle changes in combination with drug therapy. In some cases surgical intervention is used. Surgical intervention is aimed at strengthening and tightening the LES the antireflux barrier.
Achalasia is a rare disease of the esophagus muscles resulting in an inability of the LES to relax and open to let food pass into the stomach (FIGS. 7A-C). The esophagus contains both muscle and nerves. The nerves coordinate the relaxation and opening of the sphincters as well as the peristaltic waves in the esophagus. In aclasia, nerve cells located between the esophageal muscle layers are damaged. Individuals with achalasia have difficulty swallowing food. Botulinum toxin can be effective in improving symptoms of achalasia. Dilation using various dilator systems, such as balloon dilators, are also used to weaken the LES and allow food to pass.
Gastroparesis is a disorder in which the stomach takes too long to empty its contents. Gastroparesis happens when nerves of the stomach which control the movement of food through the digestive tract are damaged or stop working. The vagus nerve controls the movement of food through the digestive tract. If the vagus nerve is damaged, the muscles of the stomach and intestines do not work normally, and the movement of food is slowed or stopped. Gastroparesis most often occurs in people with type 1 diabetes or type 2 diabetes. However, up to one third of patients with gastroparesis have no identifiable cause of the disorder. Gastroparesis primarily is addressed by treating the underlying diseases causing gastroparesis. Medications to promote gastric emptying are also used. Endoscopically placed gastrostomy tubes can also be used to help drain the stomach.
Irritable bowel syndrome (IBS) is a common intestinal disorder that affects a person's colon and cause recurrent abdominal cramps, bloating, constipation, and diarrhea. The causes of IBS are not clear, but it is believed that a combination of factors can lead to symptoms of IBS, including visceral hypersensitivity, altered bowel motility, neurotransmitters imbalance, infection and psychosocial factors. There is no cure for IBS, but the symptoms may be treated by changing eating habits, reducing stress, and making lifestyle changes. Medications can also be taken to relieve diarrhea or constipation.
Other common GI tract disorders include those of the stomach and intestine, such as diarrhea and constipation. With diarrhea, muscle contractions move the contents of the intestines along too quickly and there isn't enough time for water to be absorbed. Constipation is the opposite, with the contents of the large intestines not moving along fast enough. Gastritis is an irritation and inflammation of the stomach lining by acids produced in the stomach.
While many gastrointestinal disorders can be treated with lifestyle changes and/or medications, some conditions may require surgery. One method for accessing the gastrointestinal tract include using open surgical techniques, which requires making a long incision down the center of the abdomen. Laparoscopic procedures are minimally invasive procedures wherein small “keyhole” incisions are made in the abdomen. Generally, three of more small (5-10 mm) incisions are made in the abdomen to provide access ports to various surgical instruments and a laparoscope, which transmits pictures on a video monitor. A person undergoing a laparoscopic procedure may experience less pain and scarring after surgery, and a more rapid recovery than with an open surgical procedure.
The gastrointestinal tract can also be accessed through the mouth (upper GI endoscopy) or anus (lower GI endoscopy: colonoscopy, sigmoidoscopy, enteroscopy) by inserting a flexible endoscope through the mouth or anus body cavity to the site of interest. Upper endoscopy involves the examination of the lining of the esophagus, stomach, and duodenum (first part of the small intestine) by insertion of a flexible endoscope through the mouth to the site. Lower endoscopy involves examination of the lining of the large intestine (colon). A conventional endoscope is shown in FIG. 9. The light source illuminates the pathway and site, while the camera transmits images to a monitor. Various instruments can be introduced through the instrument port.